208 citations · 1.1k across the 23 of their papers we have counts for
23 papers
Zeeman-Doppler imaging of five young solar-type stars
T. Willamo, J. J. Lehtinen, T. Hackman +5
The magnetic activity of the Sun changes with the solar cycle. Similar cycles are found in other stars as well, but their details are not known to a similar degree. Characterising…
Compressible test-field method and its application to shear dynamos
Maarit J. Käpylä, Matthias Rheinhardt, Axel Brandenburg
In this study we present a compressible test-field method (CTFM) for computing effect and turbulent magnetic diffusivity tensors, as well as those relevant for mean ponderomoti…
Modeling the interplay between epidemics and regional socio-economics
Jan E. Snellman, Rafael A. Barrio, Kimmo K. Kaski +1
In this study we present a dynamical agent-based model to investigate the interplay between the socio-economy of and SEIRS-type epidemic spreading over a geographical area, divided…
Scalable communication for high-order stencil computations using CUDA-aware MPI
Johannes Pekkilä, Miikka S. Väisälä, Maarit J. Käpylä +2
Modern compute nodes in high-performance computing provide a tremendous level of parallelism and processing power. However, as arithmetic performance has been observed to increase…
Generation of mean flows in rotating anisotropic turbulence: The case of solar near-surface shear layer
A. Barekat, M. J. Käpylä, P. J. Käpylä +2
The radial gradient of the rotation rate in the near-surface shear layer (NSSL) of the Sun is independent of latitude and radius. Theoretical mean-field models have been successful…
A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions
Jyri J. Lehtinen, Maarit J. Käpylä, Nigul Olspert +1
The magnetic activity of late-type stars is correlated with their rotation rates. Up to a certain limit, stars with smaller Rossby numbers, defined as the rotation period divided b…
Estimating activity cycles with probabilistic methods II. The Mount Wilson Ca H&K data
N. Olspert, J. Lehtinen, M. J. Käpylä +2
Debate over the existence of branches in the stellar activity-rotation diagrams continues. Application of modern time series analysis tools to study the mean cycle periods in chrom…
Estimating activity cycles with probabilistic methods I. Bayesian Generalised Lomb-Scargle Periodogram with Trend
N. Olspert, J. Pelt, M. J. Käpylä +1
Period estimation is one of the central topics in astronomical time series analysis, where data is often unevenly sampled. Especially challenging are studies of stellar magnetic cy…
Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars
Mariangela Viviani, Jörn Warnecke, Maarit J. Käpylä +5
We seek to understand the transition from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation using 3D numerical simulat…
Extended subadiabatic layer in simulations of overshooting convection
Petri J. Käpylä, Matthias Rheinhardt, Axel Brandenburg +5
We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (C…
Convection-driven spherical shell dynamos at varying Prandtl numbers
P. J. Käpylä, M. J. Käpylä, N. Olspert +2
(abidged) Context: Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims: We study the dynamo and differential rotatio…
Robustness of oscillatory dynamos in spherical wedges
E. Cole, A. Brandenburg, P. J. Käpylä +1
We study the connection between spherical wedge and full spherical shell geometries using simple mean-field dynamos. We solve the equations for a one-dimensional time-depende…
Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
Jörn Warnecke, Matthias Rheinhardt, S. Tuomisto +3
We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory. We simulate a solar-type star in a wedge-shaped spherical s…
Magnetic flux concentrations from turbulent stratified convection
P. J. Käpylä, A. Brandenburg, N. Kleeorin +2
(abridged) Context: The mechanisms that cause the formation of sunspots are still unclear. Aims: We study the self-organisation of initially uniform sub-equipartition magnetic fiel…
The Maunder minimum (1645--1715) was indeed a Grand minimum: A reassessment of multiple datasets
Ilya G. Usoskin, Rainer Arlt, Eleanna Asvestari +13
Aims: Although the time of the Maunder minimum (1645--1715) is widely known as a period of extremely low solar activity, claims are still debated that solar activity during that pe…
Multiple dynamo modes as a mechanism for long-term solar activity variations
Maarit J. Käpylä, Petri J. Käpylä, Nigul Olspert +4
Solar magnetic activity shows both smooth secular changes, such as the Grand Modern Maximum, and quite abrupt drops that are denoted as Grand Minima. Direct numerical simulations (…
Influence of a coronal envelope as a free boundary to global convective dynamo simulations
Jörn Warnecke, Petri J. Käpylä, Maarit J. Käpylä +1
We explore the effects of an outer stably stratified coronal envelope on rotating turbulent convection, differential rotation, and large-scale dynamo action in spherical wedge mode…
Multiperiodicity, modulations and flip-flops in variable star light curves III. Carrier fit analysis of LQ Hya photometry for 1982-2014
Nigul Olspert, Maarit J. Käpylä, Jaan Pelt +4
We study LQ Hya photometry for 1982-2014 with the carrier fit (CF) -method and compare our results to earlier photometric analysis and recent Doppler imaging maps. We first utilize…
On the cause of solar-like equatorward migration in global convective dynamo simulations
Jörn Warnecke, Petri J. Käpylä, Maarit J. Käpylä +1
We present results from four convectively-driven stellar dynamo simulations in spherical wedge geometry. All of these simulations produce cyclic and migrating mean magnetic fields.…
Magnetically controlled stellar differential rotation near the transition from solar to anti-solar profiles
Bidya Binay Karak, Petri J. Kapyla, Maarit J. Kapyla +3
Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and a…
Quenching and anisotropy of hydromagnetic turbulent transport
Bidya Binay Karak, Matthias Rheinhardt, Axel Brandenburg +2
Hydromagnetic turbulence affects the evolution of large-scale magnetic fields through mean-field effects like turbulent diffusion and the effect. For stronger fields, these eff…
Confirmation of bistable stellar differential rotation profiles
P. J. Käpylä, M. J. Käpylä, A. Brandenburg
(abridged) Context: Solar-like differential rotation is characterized by a rapidly rotating equator and slower poles. However, theoretical models and numerical simulations can resu…
Testing turbulent closure models with convection simulations
J. E. Snellman, P. J. Käpylä, M. J. Käpylä +2
We compare simple analytical closure models of homogeneous turbulent Boussinesq convection for stellar applications with three-dimensional simulations. We use simple analytical clo…